Stair tread are commonly used in vast numbers in residential and commercial facilities, such as homes, apartments, stores and publicly accessible structures. Many materials are utilized in stair tread construction including woods, metals, stone, concrete, and others. Stair treads are used in both indoors and outdoors applications. The foregoing uses of stair treads share a common attribute in that the environmental considerations are relatively benign. Loading of such stair treads is largely limited to normal foot traffic.
Industrial applications of stair treads, such as in factories, industrial plants, shops, chemical plants, marine facilities, and etc., are more severe. In addition to heavier structural loads, including both foot traffic and equipment, industrial stair treads may be subject to shock loads and greater extremes of heat and moisture. Also, chemicals, both caustic and acidic, may corrode stair tread materials. Abrasive materials may also play a role in aggravating the wear and tear by abrasion and erosion on such stair treads.
A typical stair case installation includes a pair of stringer beams ("stringers") connecting a first level surface to a second level surface. The stingers are typically inclined at an angle suitable for the placement of stair treads of such rise and run to accommodate a normal walking gate to climb or descend the stair case. Stair treads are disposed between the stringers and are attached by various means to securely locate the stair treads. Stringer beams are designed of a structural cross sections and from a material so as to provide adequate structural support for the span and maximum design loading of the stair case which they form. The stair treads span a distance between the stringer beams that may range from as little as perhaps sixteen inches to four feet or more. A typical stair tread width being approximately thirty-six inches.
In industrial applications, stringers are often fabricated from structural steel "C" channel or other fabricated sections. Stair treads are commonly fabricated from several materials and are of several designs, including mild steel plates, mild steel bar grating, concrete, fiberglass reinforced plastic ("FRP"), also aluminum and stainless steel are used. In the design of industrial stair treads, several criteria need to be considered. These criteria include: the cost of the material; the strength of the material; the weight of the material; the material's resistance to wear and abrasion; the material's resistance to corrosion; the material's reaction to extremes of temperature, particularly fire; the ability of the material to provide traction to the user of the stair tread; the ability of the stair tread to remain clean; the characteristics of the stair tread as it wears out, aesthetics, and other considerations.
For industrial applications, mild steel plate is often chosen as a stair tread material because it is a relatively inexpensive material and its characteristics in fabrication are well known and developed. Sometimes, so called `diamond` or `checker` plate are used for better traction on the walking surface. However, mild steel plate has several limitations in that it is relatively heavy, prone to corrosion, and provides poor traction, especially as it wears. Also, the plate must be relatively thick to provided adequate strength on wider stair tread applications. Mild steel plate tends to accumulate dirt and debris on its surfaces, further reducing traction and increasing the safety risk to users.
Mild steel bar grating stair treads, which are also commonly used, are an improvement over mild steel plate. Bar grating stair treads utilize a plurality of mild steel bars that act as beams which span the width of the stair tread. These beams are interconnected with a plurality of brace bars which locate and reinforce the beams. Together, these beams and braces form a grate structure which is strong. The openings in the grate structure make the tread self cleaning and afford a degree of visual access through a stair case. The fabrication costs of bar grating stair treads is increased as compared to plates. At each intersection of the beams and braces, a connection, typically a weld, must be made. Such a stair tread is also comprised of a large number of individual pieces which must be fabricated in turn. Also, bar grating stair treads suffer from the corrosion and weight problems, such as are related to all mild steel stair treads.
In an effort to overcome the corrosion problems related to mild steel, while retaining the advantages of a bar grating stair tread, others have devised fiberglass reinforced plastic ("FRP") stair treads. Such stair treads are fabricated in a form, or mold, into which fiberglass roving is laid into place. Each beam and brace in the grate must have fiberglass roving. The mold is then filled with plastic or epoxy and cured to form the FRP grate stair tread. Such treads do yield some of the advantages and benefits of steel bar grating treads including the self cleaning nature and general advantages of a grate design. The desired resistance to corrosion is also achieved by use of plastic material. However, FRP stair treads suffer from some important disadvantages which include: high fabrication costs due to the complex nature of the design; low production yields due to the slow fiberglass roving laying process and slow cure times; relatively high weight due to the need to use large bar cross sections necessary to achieve the needed strength, and inadequate resistance to fire exposure. In fact, fiberglass stair treads will completely fail after exposure to fire.
FRP stair treads have further disadvantages during installation. The attachment of the stair tread to the stringer requires the use of nuts, bolts, washers, angle clips, or other brackets in order to adequately support the stair tread. It is not sufficient to merely drill a hole and insert a bolt through the outermost brace because each individual brace does not exhibit adequate strength to support the load of the whole stair tread. Rather, a support shelf must be utilized to distribute the loading over a plurality of the beams/braces in the FRP stair tread. Since one of the goals is corrosion resistance, these attachment items are usually made from stainless steel where harsh chemical environments are involved. This can result in a very expensive stair tread installation, as compared to mild steel designs.
It should be noted that corrosion resistance can be achieved in a metal plate or metal bar grating stair tread by substituting stainless steel for mild steel. However this is rarely done in practice due to the extremely high costs of both material and fabrication in stainless steel.
Clearly there is a need for an industrial stair tread which is low cost, strong, corrosion resistant, withstands wear, is serviceable during or after fire, is easily fabricated in high volume, and offers the various advantages of a bar grating and/or plate type stair tread designs.